DIFFUSION AND OSMOSIS 



57 



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diffusion between two poles of the same 

 sign. 



The positive and negative poles and the lines of force in 

 the field of diffusion may he illustrated by the following 

 experiment. A thin layer of salt water is spread over an 

 absolutely horizontal plate of glass. If now we take a drop 

 of blood, or of Indian ink. and drop it carefully into the 

 middle of the salt solution, we shall find that the coloured 

 particles will travel along the lines of diffusive force, and thus 

 map out for us a monopolar 

 field of diffusion, as in Fig. 

 3 a. Again, if we place two 

 similar drops side by side in 

 a salt solution, their lines 

 ol' diffusion will repel one 

 another, as in Fig. 4. ' >f' >'...-" 



Now let us put into the 

 solution, side by side, one FIG. 4.— Two drops of blood in a more 

 drop of less concentration 

 and another of greater con- 

 centration than the solution. 

 The lines of diffusion will pass from one drop to the other, 

 diverging from the centre of one drop and converging to- 

 wards the centre of the other (Fig. 8b). In this manner we 

 are able to obtain diffusion fields analogous to the magnetic 

 fields between poles of the same sign and poles of opposite signs. 



The conception of poles of diffusion is of the greatest 

 importance in biologv, throwing a flood of light on a number 

 of phenomena, such as karyokinesis, which have hitherto been 

 regarded as of a mysterious nature. It also enables us to 

 appreciate the role played by diffusion in many other 

 biological phenomena. Consider, for example, a centre of 

 anabolism in a living organism. Here the molecules of the 

 living protoplasm are in process of construction, simpler 

 molecules being united and built up to form larger and more 

 complex groups. As a result of this aggregation the number 

 of molecules in a given area is diminished, i.e. the concentra- 

 tion and the osmotic pressure fall, producing a hypotonic 

 centre of diffusion. We may thus regard every centre of 

 anabolism as a negative pole of diffusion. 



